The view from the top of is great! After walking around Kyoto for four days and being very tired in the feet, I adopted the idea of some Japanese women who were having a pleasant conversation, and rested, enjoying the scenery and the pleasant damp air.
You can also visit the temple and the gardens, for a fee. The garden look pretty small; not sure if it would be worth it. Nanzenji is very pleasant on a rainy day, since there are a lot of trees that get a saturated color in the rain.
Nanzenji is probably a 15 minute walk from Chionin, but it is not served well by bus. If you are in the area the bus will be convenient; plan to visit Nanzenji in conjunction with other locations in the area.